101.对称二叉树

//1、递归
var isSymmetric = function (root) {
  if (!root) return false;
  let flag = true;
  const rec = (left, right) => {
    if (left === null && right === null) {
      return;
    }
    if (left === null || right === null || left.val !== right.val) {
      flag = false;
      return;
    }
    rec(left.left, right.right);
    rec(left.right, right.left);
  };
  rec(root.left, root.right);
  return flag;
};

//2、迭代

var isSymmetric = function (root) {
  if (!root) return false;
  const stack = [root.left, root.right];
  while (stack.length) {
    let p1 = stack.pop();
    let p2 = stack.pop();
    if (p1 === p2) continue;
    if (p1 && p2 && p1.val === p2.val) {
      stack.push(p1.left, p2.right, p1.right, p2.left);
    } else {
      return false;
    }
  }
  return true;
};